In order for a communications device to successfully complete a transmission, a collision should not occur with transmissions from other communications devices or with noise and other types of interference. It may be possible to recover from a collision with another transmission (or noise or other types interference) via the use of error detecting and correcting codes. However, such recovery is normally possible only if a small percentage of the transmission contains unreliable bits or information, such as a result of transient noise or interference, and not when a large percentage of the transmission is damaged, which is normally the case when a collision with another transmission occurs. Furthermore, the use of error detecting and correcting codes that can recover from significant damage can greatly reduce the overall throughput performance of a communications system due to the coding overhead.
One way to help reduce the probability of transmission collisions is to divide the available bandwidth in the communications channel into multiple transmission bands and then assign certain communications devices to the various transmission bands, wherein the communications devices can only transmit within their assigned transmission bands. By assigning communications devices to different transmission bands, the probability of collisions with other transmissions can be reduced. If the number of transmission bands is equal to or greater than the number of communications devices, then the probability of collisions can be reduced to zero.
Another way to help reduce the probability of transmission collisions is to allocate access to the communications channel based upon time, wherein a communications device can transmit only if it is within its transmission time window. Once again, the probability of collisions can be reduced to zero if only one communications device is assigned to a transmission time window.
One disadvantage of the prior art is that if the available bandwidth is divided into a large number of transmission bands, then the total amount of bandwidth available to a single communications device can be a small fraction of the total bandwidth. If only a small number of communications devices are transmitting, then the bandwidth utilization can be small, resulting in the waste of a significant amount of the available bandwidth.
A second disadvantage of the prior art is that unless properly allocated, the transmission time windows can be allocated to communications devices with nothing to transmit, while communications devices with a need to transmit may not receive enough transmission time windows to achieve adequate data throughput. Once again, this can lead to inefficient use of the available transmission bandwidth.
Another disadvantage of the prior art is that should an adaptive technique be applied to either the transmission bands or the transmission time windows, to increase bandwidth utilization, for example, then considerable resources may be needed to fairly and effectively distribute available bandwidth. This may require the use of a dedicated bandwidth server (or processor), which can increase the costs of the communications system.